1. Field of the Invention
The present invention relates to a flat display and a process for producing a cathode plate for use in the flat display. The cathode plate according to the present invention comprises minute field emission cathodes and can ensure a higher electron emission efficiency and a higher luminosity than a conventional thermionic cathode, and is a promising electron source for a flat display, an image pickup tube and the like. Particularly, a display device employing such minute field emission cathodes is of a self-luminous type, and can exhibit a higher luminance and a higher resolution. Further, the display device has excellent characteristics (e.g., wide view angle, quick response and low power consumption).
2. Description of Related Art
FIG. 14 shows one exemplary construction of a display device employing a conventionally known minute field emission cathode.
The minute field emission cathode includes conical emitter tips 101 each having a sharp tip, gate electrode lines (gate power supply lines) 103 for extracting electrons, emitter electrode lines (emitter power supply lines) 102 for applying a negative voltage to the emitter tips, and an insulation layer 104 for isolating the gate electrode lines from the emitter electrode lines. The minute field emission cathode is disposed on a glass substrate 105 as shown in FIG. 14. The entire cathode structure including the glass substrate is herein called a cathode plate 109. Several hundred to several thousand emitter tips are formed on the emitter electrode lines 102 within each of intersection areas where the emitter electrode lines 102 intersect the gate electrode lines 103.
One display element (e.g., pixel) 106 comprises these several hundred emitter tips.
A voltage is applied between the emitter tips 101 and the gate electrode lines 103, and electrons are extracted from the emitter tips 101 into a vacuum space by field emission. The emitter tips 101 each having a micron size are formed at a high integration density by utilizing semiconductor microprocessing techniques.
The field emission characteristic is non-linear, so that the emitter electrode lines and the gate electrode lines can be driven by simple matrix addressing. Electrons extracted from emitter tips in a selected pixel impinge on a transparent anode plate 107 which is spaced several hundred micrometers from the minute field emission cathode in an opposed relation.
The anode plate 107 includes fluorescent layers 108 formed in a stripe pattern on its surface. The fluorescent layers 108 are excited to emit light when electrons impinge on the fluorescent layers 108. The light emission is observed through the anode plate 107 from the top of FIG. 14 by a user.
FIGS. 15(a) to 15(h) show a process for producing matrix elements of the minute field emission cathode by a conventionally employed vacuum deposition.
An emitter power supply film 117 is formed on an insulative substrate 116 such as of a glass as shown in FIG. 15(a), and patterned into emitter electrode lines 102 as shown in FIG. 15(b).
Subsequently, an insulating film 118 and a gate power supply film 119 are formed in this order over the resulting substrate as shown in FIG. 15(c).
As shown in FIG. 15(d), the gate power supply film 119 and the insulating film 118 are respectively etched with the use of a resist pattern having circular openings for formation of cylindrical gate openings 120.
As shown in FIG. 15(e), a sacrificial film material such as aluminum is deposited on the resulting substrate obliquely with respect to the insulative substrate 116 for formation of a sacrificial film 121 in such a manner that the material is not deposited on portions of the emitter power supply film 117 exposed within the gate openings 120.
As shown in FIG. 15(f), an emitter tip material 122 such as molybdenum is deposited on the resulting substrate vertically with respect to the insulative substrate 116. As the emitter tip material is deposited with time, the gate openings 120 are gradually covered with the emitter tip material. When the gate openings 120 are completely covered, conical emitter tips 101 are formed within the respective gate openings 120 as shown in FIG. 15(f).
In turn, portions of the emitter tip material 122 except the emitter tips 101 are removed by selectively etching the sacrificial film 121 with an aqueous solution of phosphoric acid as shown in FIG. 15(g).
Finally, the gate power supply film 119 is patterned into a desired configuration as shown in FIG. 14 for formation of gate electrode lines 103. Thus, the minute field emission cathode is completed as shown in FIG. 15(h).
The display device shown in FIG. 14 is constructed such that the cathode plate 109 including the minute field emission cathode formed on the glass substrate 105 is bonded to the anode plate in an opposed relation with spacers (not shown) interposed therebetween for keeping a spacing of several hundred micrometers therebetween. The light emission from the fluorescent layers is viewed through the anode plate 107 from the top of FIG. 14.
Although the minute field emission cathode itself is capable of emitting electrons at a high current density in a very high vacuum, the conventional display device fails to ensure a high luminosity because of a low luminous efficiency of the fluorescent layers.
Further, an acceleration voltage to be applied to the anode 107 is limited to several hundred volts at the maximum because of the small spacing between the cathode plate 109 and the anode plate 107. Fluorescent materials which can be excited for light emission by electron beams accelerated at a voltage of several hundred volts have been put in practical use for fluorescent display tubes, but only green fluorescent materials have sufficiently high luminous efficiencies.
Since a low speed electron beam can penetrate through only a several-atom surface layer of a fluorescent layer, only the surface layer emits light. In the case of the display device having the conventional construction shown in FIG. 14, the light emission from the surface layer of the fluorescent layer is observed through the anode plate 107 (including the fluorescentlayers 108 and the glass substrate) from the rear side thereof. Since the light is scattered toward the side of the cathode plate 109 by the fluorescent layers 108, a luminosity observed from the rear side of the anode plate is lower than that observed from the side of the cathode plate.
FIG. 16(a) is a schematic view illustrating one exemplary construction of the conventional anode plate 107. As shown, a user views light emission through a fluorescent layer 108 and the glass substrate from a position FLVFD and, therefore, the luminosity is lower than when the scatter light is directly viewed from a position VFD.
As shown in FIG. 16(b), where the fluorescent layer is formed of ZnO:Zn and has a thickness of about 10 .mu.m (i.e., a thickness equivalent to the thickness of a one- or two-particle layer of a fluorescent substance), the luminosity observed from the position FLVFD is the maximum but only about 60% of that observed from the side of the light emission face (from the position VFD). Further, the formation of such a thin fluorescent layer requires a highly advanced technique.
On the other hand, the luminance of the fluorescent layer can be increased by increasing the density of current to be applied thereto, because the luminance is directly proportional to the product of the luminous efficiency of the fluorescent layer and an electric power (the product of a current density and an acceleration voltage) applied to the fluorescent layer. However, the luminous efficiency levels off even if the current density is increased. In addition, the excitation of the fluorescent layers at a high current density shortens the lifetime of the fluorescent layers.
If the acceleration voltage to be applied to the anode plate is increased to the order of several kilovolts, the required current density can be reduced so that a fluorescent material adapted for high speed electron beam excitation for used in CRTs can be employed. The application of a higher acceleration voltage requires a higher insulation withstand voltage which is attained by increasing the gap (spacing) between the cathode plate and the anode plate. However, spacers having a high aspect ratio are required for increasing the gap without increasing a pixel pitch, and the formation of such spacers is not easy. Further, the increase in the gap between the cathode plate and the anode plate results in image blurring and color diffusion, because electron beams from the cathode diffuse to adjacent pixels. Therefore, an additional electrode for converging electron beams is required.
In the conventional display device shown in FIG. 14, a getter for adsorbing gas particles emanating from the fluorescent layers 108 and the like is required, from a structural viewpoint, to be provided in an area separate from a display area where the pixels are provided. More specifically, the getter accommodating area is located in a position remote from the display area. Therefore, a greater amount of the getter and a larger getter accommodating area are required as the display area is increased.
Unlike the CRTs and fluorescent display tubes, the display device employing the minute field emission cathode has a very small ventilation conductance and, therefore, it requires much time for the getter to adsorb a gas emanating from the fluorescent layers due to electron bombardment and a gas emanating from the cathode plate and the spacers due to ion bombardment. The vacuum level around the emitter tips is locally reduced with time due to the gas emanating from the fluorescent layers. The reduction in the vacuum level causes arc discharge, resulting in destruction of the emitter tips.
The reduction in the vacuum level, though not causing arc discharge, reduces the electron emission from the emitter tips. In general, the value of the emission current from the emitter largely depends on the vacuum level around the emitter. This is because oxygen molecules and water molecules in a residual gas are adsorbed on the surface of the emitter to increase a work function on the emitter surface. That is, a display device constructed such that getters are provided along edges of the display panel has a problem associated with the lifetime thereof. The distances between the getters and emitter tips located in a central portion of the display area are increased as the display area is increased. This aggravates the lifetime problem.
Japanese Laid-Open Patent No. Hei 5(1993)-94787 has disclosed a fluorescent display apparatus which has an electric field emission element (FEC), and a light transmissive part is formed between FECs. U.S. Pat. No. 5,223,766 has disclosed a image display device with cathode panel and gas absorbing getters.